The cellular radio communication networks are currently diversifying in order to support not only the conventional voice, video and internet services but also the increasing traffic of communications from machines or things. The internet of things IoT is booming and addresses a multitude of fields of applications, some of which demand a service of high reliability over global coverage. Thus, in the transport domain, the communications between machines make it possible to remotely track and monitor merchandise routed by sea, air and/or land.
The issue in communications between machines is therefore to offer ubiquity of service at lower cost and with the fastest possible infrastructure deployment time.
This ubiquity of service is also an issue for certain critical low bit rate applications which require high service availability, such as security telecommunications for example.
Thus, the integration of a satellite component in the terrestrial cellular networks is sought to meet the need for ubiquity of these new IoT services characterized more often than not by sporadic data traffic at low bit rate transmitted in bursts.
The integration of a satellite component is a solution that is well known to meet the need for ubiquity of the conventional voice, video and internet services.
A first family of current satellite communication solutions is based on the use of geostationary satellites, which offer the global coverage and the service reliability required for these conventional services. However, in comparison to the terrestrial networks, the geostationary network infrastructures are also known for their high service cost and terminal costs and for a relatively mediocre performance in terms of quality of service and of latency.
The degradation of the latency is linked to a significant information transit time due to the distance separating the satellite from the surface of the earth, which can negatively impact the quality of service for the real-time applications.
The degradation of the quality of service for a geostationary satellite system can also be due to a lack of space diversity and to the stationary nature of the propagation path between a user terminal and the satellite. When a propagation path is subject to strong attenuations, even blocked, and if the terminal is fixed, these propagation conditions will not change. The data cannot be transmitted, without moving the terminal itself.
A second family of current satellite communication solutions is founded on the use of satellites organised in constellations and moving in non-geostationary or non-geosynchronous orbits NGSO. Among the NGSO systems, the satellite systems using satellites in low earth orbits LEO, such as the commercial global star or iridium systems, make it possible to reduce the latency through a low transit time and improve the quality of service through a dynamic diversity of the propagation conditions offered by the satellites in view of the LEO constellation.
In these two families of solutions, the service cost is linked to the limited capacity of the infrastructure and of the spectrum allocated to the mobile satellite service MSS accessible in particular below 3 GHz.
In these two families of solutions, the terminals are dual-mode terminals, in as much as they are configured to communicate by choice with the satellites via a first wireless interface or with the relay stations of a terrestrial cellular network via a second wireless interface.
Notwithstanding the fact that the relatively limited market, addressed by these commercial satellite systems does not incentivize the terminal manufacturers to diversify their range of products, or seek to reduce their costs and/or their footprints a first variant and a second variant of integration of the satellite component and of the terrestrial cellular component for supplying conventional services have been proposed in order to make the production of the dual-mode terminals more attractive.
According to the first variant, the radio interface of the cellular network is re-used in the satellite system by making the links between the terminals and the satellites operate in a specific frequency band, close to the much wider MS (Mobile Service) service band allocated to the terrestrial cellular system. The impact on the terminals is thus limited to the radio stage which must if necessary extend its frequency band, even with a few protocol modifications (for example modifications concerning the power control protocol, the synchronisation mechanism, the contention-based access protocol) to allow optimal operation in the geostationary satellite system. Such a variant is described by the international telecommunications union ITU in the document published under the reference ITU-R recommendation M.2047-0(12/2013), entitled “Detailed specifications of the satellite radio interfaces of International Mobile Telecommunications—Advanced (IMT Advanced)”.
It is noteworthy that this first variant can be generalised to two infrastructures of a system which use different spectra and the access to the spectrum is without mutual constraint.
According to the second variant, an integrated satellite/cellular system operating in the MSS (Mobile Satellite Service) frequency bands allocated specifically to the mobile satellite services could be developed. However, it is not possible to simultaneously use the same MSS satellite frequency band (Mobile Satellite Service band) on one and the same coverage area shared by the satellite component and the terrestrial cellular component. The risk is that of creating satellite service exclusion areas around the terrestrial base stations or of reducing the effective range of the base stations. It is then accepted practice to provide a frequency re-use scheme at the satellite level which then constrains the use of the spectrum at the terrestrial cellular level. The management of the spectrum of this system thus has to be under the control of satellite operators, which does not incite the support of the terrestrial cellular operators and, consequently, does not interest the manufacturers of cellular terminals.
It is noteworthy that this second variant can be generalized to a system in which a lower level re-uses the spectrum not used by a higher level under the control of the operator of the higher level.
Thus, according to these two variants, the size of the accessible market or the operation conditions do not incite the manufacturers of terminals to launch into the production of these dual-mode terminals despite a relatively low increase in the proportion of the recurrent production costs of the dual-mode terminals.
However, here, and unlike the conventional services in which the traffic load prioritizes the traffic volume on the downlink over the uplink and the downlink, the new internet of things IoT services for which the seamless extension of the coverage is sought are characterized by an asymmetrical traffic load, mostly on the uplink, and an uplink traffic component obeying sporadically a low bit rate data burst transmission law.
Here, the aim is to determine, in this case, an architecture of a satellite system which allows for a seamless extension of the coverage area of the terrestrial cellular networks, in particular of the fourth generation 4G networks or of the fifth generation 5G networks, for communications between things and a maximum use of the spectrum allocated to the terrestrial cellular networks outside of the totality of the coverage areas of the terrestrial cellular networks.
Generally, the technical problem is how to increase the transmission capacity on the uplink of a radio communication system with one or more umbrella cells, integrated in terms of complementing coverage, with one or more terrestrial cellular radio communication systems with cells of smaller size, and configured to route asymmetrical data traffic corresponding to communications between things, loaded mostly on the return channel of the radio communication system with umbrella cells.
Additionally, a second technical problem is how to reduce the complexity of the dual-mode terminals capable of selectively accessing the radio communication system with umbrella cells and terrestrial cellular networks as a function of its geographic position and/or observed propagation conditions.